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IC 


8944 



Bureau of Mines Information Circular/1983 




Computerized, Remote Monitoring 
Systems for Underground Coal Mines 



Faults in Power Systems 



By Jeffrey H. Welsh 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8944 



Computerized, Remote Monitoring 
Systems for Underground Coal Mines 



Faults in Power Systems 



By Jeffrey H. Welsh 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 




Library of Congress Cataloging in Publication Data: 



Welsh, Jeffrey H 

Computerizjed, remote monitorinj; systems for underground coal 
mines. Faults in power systems. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 8944) 

Bibliography: p. 8, 

Supt. of Docs, no.: I 28.27:8944. 

1. F.lectricity in mining— Safety measures— Data processing. 2. 
Coal mines, and mining— Safety measures— Data processing. 3. C-CRMMS 
(Computer system). 4. Ivlectric fault location— Data processing. I. Ti- 
tle. II. Series: Information circular (United States. Bureau of Mines) ; 
8944. 

JLN295JJ4 ITN3431 622s [622'. 48'0289] 83-600132 












CONTENTS 

Page 



Abstract 1 

^ Introduction 1 

.^ Need for improvement In power system safety 2 

K} Types of power system faults 2 

Monitoring power system faults 2 

Analysis of regulations 4 

Systems currently Installed 6 

Production benefits 7 

Conclusions 7 

References 8 

Appendix. 9 

ILLUSTRATIONS 

1 . Mine monitoring system block diagram. 3 

2. Underground power system distribution 4 

TABLES 

1 . Fatalities in underground coal mines 2 

2. Functions of a mine monitoring system 5 

3. Causes of downtime 7 

A-1 . Current mandatory standards 9 





UNIT OF MEASURE ABBEIEVIATIONS USED IN THIS REPORT 




A 


ampere min 


minute 


ft 


foot pet 


percent 


ml 


mile V 


volt 



COMPUTERIZED, REMOTE MONITORING SYSTEMS FOR UNDERGROUND COAL MINES 

Faults in Power Systems 

By Jeffrey H. Welsh ^ 



ABSTRACT 

The Bureau of Mines studied the use of computerized, continuous, re- 
mote monitoring systems for safety from power system faults in under- 
ground coal mines. In this report the need to improve protection 
against power system faults is documented, and types of faults are 
identified. The relationship between mine safety regulations and com- 
puterized, continuous, remote monitoring is analyzed. 

INTRODUCTION 

Electrical power systems are important in underground coal mines. 
Ventilation fans, pumps, compressors, hoists, and battery chargers use 
electricity. It is also used by coal mining equipment (continuous 
miners , roof bolters , shuttle cars , cutting machines , longwalls , and 
loaders). Electricity runs both the conveyors and railroads used in 
transporting coal out of the mines. Mine electrical systems vary with 
the method of mining and the type of haulage. Faults in these power 
systems not only cause many accidents each year but also effect a de- 
crease in production. 

Computerized, continuous, remote mine monitoring systems (CCRMMS) can 
improve safety in underground coal mines and increase production. 
Equipment is available commercially to monitor power system faults. 
This equipment includes (1) sensors that detect impending power system 
faults and (2) microprocessor-based computers that collect and analyze 
the data from these sensors. 



'Operations research analyst, Pittsburgh Research Center, Bureau of Mines, Pitts- 
burgh, PA. 



NEED FOR IMPROVEMENT IN POWER SYSTEM SAFETY 



Electricity injures twice as many pro- 
duction workers as maintenance workers. 
Most (4^)2 electrical accidents are caused 
by arcs, electrically generated heat, 
or shock. Table 1 (2^, T) shows that 
electrical accidents are the fourth lead- 
ing cause of fatalities in underground 
mines. In 1981 (2^) there were 246 in- 
juries caused by electric current that 
resulted in days away from work. MSHA 
citations {V) show that Mandatory Safety 
Standards for electrical equipment have 
twice as many violations as any other 
category. 

Fires can be related to electrical 
faults. Electrical ignitions started 47 
pet {b) of the mine fires reported from 
1970 to 1980. Explosions also can be re- 
lated to electrical faults. Electricity 
was the source of 21 pet (4^) of methane 
ignitions investigated through 1972. The 
CCRMMS can improve safety from power sys- 
tem faults. 



TABLE 1. - Fatalities in underground 
coal mines 



Causes 

Fall of roof 

Powered haulage 

Machinery 

Electrical 

Explosives and breaking 

agents 

Ignition or explosion 

of gas or dust 

Falling, rolling, or 

sliding material 

Hoisting 

Fall of face, rib, 

side, or highwall. , . . , 

Handling material 

Inundation 

Hand tools 

Other 

Total 



1979 



62 

24 

10 

8 





1 
3 

5 

1 
1 




115 



1980 



30 
25 
16 
10 

7 

5 

2 
2 

1 
1 


1 



100 



1981 



37 

20 

6 

9 

3 

36 


1 

4 
1 
3 

1 



121 



TYPES OF POWER SYSTEM FAULTS (1) 



A fault in a mine's power system can 
cause overloads, short circuits, under- 
voltages, or grounded conductors. Three 
types of faults can occur: quality, 
stress, and wear. 

Quality faults result from Improper 
design, bad workmanship, damage dur- 
ing transportation, and errors in 
installation. 



voltage. These faults also result from 
accidents, for example, when a continuous 
miner runs over a trailing cable or a 
roof fall damages electrical equipment. 

Wear on the power system results from 
its use and exposure to changes in the 
environment. Frictional wear, insulation 
breakdown, and corrosion are examples. 
Maintenance helps reduce wear. 



Stress faults result from increased 
stress levels or combinations of stress 
such as excessive temperature and 



Computerized monitoring can both pre- 
dict and quickly detect power system 
faults. 



MONITORING POWER SYSTEM FAULTS 



A coii^)uterized monitoring system con- 
sists of an aboveground computer that 
collects, records, and prints routine 
and alarm data from sensors in the mine. 
An alarm indicates that an abnormal 

^Underlined nxunbers in parentheses re- 
fer to items in the list of references 
preceding the appendix. 



condition exists in the mine, which needs 
immediate corrective action, such as an 
equipment failure or a methane accumu- 
lation. The computer is continuously 
manned; therefore, underground miners can 
be quickly alerted to danger. An unin- 
terruptible power supply operates the 
computer during power failures. Intrin- 
sic safety requirements must be met if 



sensors are to be placed in return air. 
Figure 1 shows a block diagram of a 
CCRMMS. 

Many miles of cable are needed to carry 
electricity throughout a mine. Because 
of the complexity of a power system, de- 
termining the location of a fault may be 
difficult. Figure 2 shows how the power 
system network is distributed. Monitor- 
ing these power system components can 
increase safety and production. Safety 
improves because power system faults or 
potential faults are rapidly detected 
and alarmed. Production improves because 
downtime is reduced when faults can be 
located quickly. 

Monitoring enables the status of cir- 
cuit breakers and sequence of circuit 
breaker trips to be determined. A short- 
to-ground in one section can trip other 
power centers. Knowing the sequence of 



the circuit breaker trip helps locate the 
origin of the problem. 

Another benefit results from monitoring 
the dc trolley-wire system. Both normal 
and unwanted load currents flow through 
the ground. An unwanted load can cause a 
fire even though it may draw less current 
than a normal load. A remotely monitored 
discriminating circuit breaker, which has 
the ability to detect an unwanted load, 
warns the mine operator of dangers. 
Again, safety increases. 

Once installed, the system can also 
monitor and control the environment, 
production, energy use, and maintenance. 
Control signals sent to equipment under- 
ground are based on monitored inputs 
or programmed time intervals. The more 
parameters the CCRMMS monitors , the 
greater the return in benefits. 



Central , manned location 



Above ground 



Backup 
power supply 



Routine 
data printer 



Alarm 
printer 



[Communications 



Micro- 
processor- 
based 
computer 



CRT 



Underground 



Belts 



Pumps 



Auxiliary 
fans 



Switch- 
gear 



Circuit 
breakers 



Electric 

face 
equipment 



Power 
center 



Fan 



.• J ■<■. 



\ Telephonel 



FIGURE 1. - Mine monitoring system block diagram. 



From 
utility company 



Surface loads 




Borehole shaft 
or slope 



[7j[« **0 



(Continuous mining 
section) 



<\i "> 'J- Ki I „,\ >o K * p> 
IT) lO If) It, [£/J lO IT) f) If) 

(Longwoll mining section) 






(Conventional mining section) 



LEGEND 

/ Utility company metering 3Z 

2 Main substation 53 

3 Surface substation 34 

I Disconnect switchhouse, mine 35 

5 Portable switchhouse, rectifier 36 

6 Power center, miscellaneous loads 37 

7 Rectifier, trolley system 38 

8 Portable switchhouse, continuous mining 39 

9 Distribution transformer, belt drive 40 

10 Power center, continuous mining 41 

II Portable switchhouse, 4Z 
distribution transformer 43 

12 Distribution transformer, individual loads 44 

13 Portable switchhouse, longwoll 45 

14 Portable switchhouse, conventional 45 

15 Power center, conventional mining ^7 

16 Rectifier, shuttle cars 4g 

17 Power center, longwoll mining 49 

18 Distribution box, longwoll 50 

19 Distribution box, longwoll 5/ 

20 Conveyor belt starter and drive 5^ 

21 Master control, longwoll jj 

22 Prep plant."?) s4 
^J Fan / 55 

24 Hoists V o . u • /,. 56 

25 Shop / 2"^*°" substation W ^^ 

26 Office I SB 

27 Pumps. . . ._^ s9 

28 Shop -1 

29 Pumps . . . ./ _ 

„ n i. , . > Power center (W 

30 Belt drive. I 

31 Bunker . . ) 



Trolley system 
Continuous miner." 
Shuttle car.. 
Shuttle car. 

Bolter , _ 

Feeder.. ) Power center^; 

Pump... 

Charger. 

Spare , . . 

Pump... . 

Conveyor..^ Distribution transformer «?; 

Spare.. 

Loader. 

Cutter . . 

Bolter V 

\«i„4», „. / Power center (75/ 

Water pump. ' 

Car spotter. 

Spare . 

Shuttle can 

Shuttle car j''«<="««^^''^^ 

Shearer 71 

Stage loader. .1 „ 

_ > Distribution box 18) 

Conveyor ( 

Water pump. .J 

Conveyor 7) 

Hydraulic pump. ( _. ,, . 
UvMronii^ n.,™^ > Distribution \>w.09) 
Hydraulic pump.i 

Spore .\ 



FIGURE 2. = Underground power system distribution. 



ANALYSIS OF REGULATIONS 



To determine the role of CCRMMS in pro- 
viding an increased safety level in 
mines, and to determine the relationship 
between mine safety regulations and 
CCRMMS, an analysis of CFR Title 30, 
Part 75, Electrical Equipment, subparts 
F, G, H, I, J, and K, was made. It 
should be noted that it is assumed by 
this analysis that the CCRMMS is opera- 
tional, available, and accurate at all 
times. 

Functions a CCRMMS can perform to en- 
hance mine safety over that provided with 
CFR Title 30, Part 75 are shown in ta- 
ble 2. Safety is increased since moni- 
toring is continuous; faults are de- 
tected, located, and diagnosed rapidly; 
and alarms are sent to a central, manned 
location aboveground. 



Sensors are commercially available to 
monitor all parameters listed in the 
Functions colximn of table 2 except: 
(a) temperature of a cable over its en- 
tire length (75.513); (b) dielectric 
strength of the insulation (75.513); and 
(c) determining fault location on dc 
trolley wires (75,1001). In these cases, 
prototypes are available. 

No lOlC petitions, in which an operator 
requests to use CCRMMS in lieu of apply- 
ing current safety requirements , have 
been filed with MSHA by mine operators 
for CFR Title 30, Part 75, subparts F, G, 
H, I, J, or K. 



I .tMbu-'.MXjar^ 



TABLE 2. - Functions of a mine monitoring system 

CFR Part and title^ Function of mine monitoring system 

Subpart F, Electrical Equipment — General: 

75.513 — Electrical conductor; capacity Monitor temperature, voltage, and current 
and installation. the cable is carrying. Check dielectric 

strength of the insulation. 

75.518 — Electric equipment and circuits; Monitor status of fuses, circuit breakers, 
overload and short circuit protection, and relaying devices leading to the cir- 
cuit interrupts. Monitor voltage, cur- 
rent, ground continuity, ground wire cur- 
rent, and phase sequence of the cable. 

75.519 — Main power circuits; disconnect- Monitor status of switches and power going 
ing switches. through them. 

75.520 — Electrical equipment; switches... Monitor status of switches. 

75.524 — Electric face equipment; electric Monitor if protecting device is in place 
equipment used in return air outby the and if the device is functional, 
last open crosscut; maximum level of al- 
ternating or direct electric current be- 
tween frames of equipment. 

Subpart G, Trailing Cables: 

75.601 — Short circuit protection of Monitor status of circuit breakers, fuses, 
trailing cables. protective relaying devices. Monitor 

maximum levels of current the cable is 
carrying. 

Subpart H, Grounding: 

75.702 — Protection other than grounding.. Monitor if ground is physically present. 

75.705 — Work on high-voltage lines; de- Monitor power, voltage, current pass- 
energizing and grounding. ing through lines. Monitor status of 

switches. 

75.706 — Deenerglzed underground power Monitor status of circuit breakers, 
circuits; idle days, idle shifts. 

Subpart I, Underground High-Voltage Distribution: 

75.800 — High-voltage circuits; circuit Monitor status of circuit breakers, 
breakers . 

75.801 — Grounding resistors Monitor continuity of grounding resistor. 



See footnote at end of table. 



TABLE 2. - Functions of a mine monitoring system — Continued 

CFR Part and title^ Function of mine monitoring system 

75.803 — Fail-safe ground check circuits Monitor if ground is physically present, 
on high-voltage resistance grounded 
systems. 

75.808 — Disconnecting devices Monitor status of switches (open or 

closed) . 

75.812 — Movement of high-voltage power Monitor status of circuit breakers, 
centers and portable transformers; 
permit. 

Subpart J, Underground Low- and Medium-Voltage Alternating-Current Circuits: 

75.900 — Low- and medium-voltage circuits Monitor status of circuit breakers; moni- 
serving 3-phase alternating-current tor current and voltage levels, 
equipment; circuit breakers. 

75.902 — Low- and medium-voltage ground Monitor status of grounds, 
check monitor circuits. 

75.903 — ^Disconnecting devices Monitor status of disconnecting devices. 

75.906 — Trailing cables for mobile equip- Monitor status of grounds, 
ment , ground wires , and ground check 
wires. 

Subpart K, Trolley Wires and Trolley Feeder Wires: 

75. 1000 — Cutout switches Monitor status of switches and relay 

devices. 

75. 1001 — Overcurrent protection Monitor current inline and status of re- 

^ lays. Determine fault location. 

'U.S. Code of Federal Regulations, Title 30. 

SYSTEMS CURRENTLY INSTALLED 



Approximately 30 U.S. coal mines have 
monitoring systems; most are recent in- 
stallations. Several of these systems 
monitor power system components. 

An example is Company A (8^) , which has 
fans and circuit breakers spread over the 
countryside, some 5 mi from the mine por- 
tal. If a fan circuit breaker trips, it 
may take considerable time for someone 
to get to the site. The reduced ventila- 
tion may cause methane to accumulate. 
This produces an explosion hazard. Also, 



miners must be withdrawn from the mine if 
ventilation cannot be restored within 15 
min. This causes a loss in production. 

Company A, therefore, monitors and re- 
sets fan circuit breakers remotely. Af- 
ter an interruption, the company resets 
the circuit breakers; if there is a sec- 
ond interruption, someone is sent to the 
site. Remote resetting of the circuit 
breakers may permit miners and equipment 
to keep working. 



I 



Company A's system also remotely trips 
circuit breakers and monitors power con- 
sumption of the face equipment. The 
ability to remotely trip circuit breakers 
provides the capability to rapidly de- 
energize power in the mine during emer- 
gencies. By analyzing time histories 
of face equipment power consumption, 



inefficiencies such as logistical bottle- 
necks , equipment problems , or sections 
that need extra help are identified. 

As the production and safety benefits 
of CCRMMS become known to the mining in- 
dustry, power system monitoring will 
increase. 



PRODUCTION BENEFITS 



Monitoring power systems can decrease 
downtime and, therefore, increase pro- 
duction. A decrease in downtime results 
from being able to rapidly locate and 
diagnose power system faults. A per- 
son with the right skills, tools, and 
repair parts can be sent to the fault 
location. 

Table 3 shows causes of downtime and 
amount of time down for 126 time stud- 
ies (_5) . Downtime costs (during cut- 
ting time, per section) used are as 
follows: 

Continuous miner - $20/min; 

Longwall - $100/min. 

It was also estimated (_5) that comput- 
erized monitoring and control could 
decrease downtime by 60 pet. This has a 
significant impact on production. Cost- 
benefit analyses (8_) have shown that com- 
puterized monitoring systems can provide 
substantial economic benefits to the mine 
operator. 



Continuous monitoring of power system 
components may also predict faults. 
Problems can be corrected on maintenance 
shifts before the fault occurs. 

TABLE 3. - Causes of downtime 





Aver- 


Downtime cost 




age 
down- 


per section 


Cause of downtime 


Cont. 






time, 


miner 


Longwall 




man 






AC power at neck of 








section 


20 


$400 


$2,000 


AC power at OCB 








submains 


24 


480 


2,400 


AC power at outside 


48.4 


968 


4,840 


Ground fault 


91 


1,820 


9,100 


DC rail haulage..,. 


23.4 


468 


2,340 


Belt knocks power.. 


20 


400 


2,000 


Fan down 


0) 


C) 


(2) 



is down 
be with- 



^Loss of shift. If the fan 
more than 15 min, miners must 
drawn from the mine. 

2 Varies depending on amount of time 
left on shift when fan goes down and on 
face equipment efficiency. 



CONCLUSIONS 



The CCRMMS can monitor many power sys- 
tem parameters. This will improve safety 
and production. 

Safety will improve because: 

(a) Power system status is monitored 
continuously. 

(b) The risk of accidents from un- 
knowns is all but eliminated. Faults in 
the power system are detected, located. 



and diagnosed rapidly. Downtime on 
equipment critical to mine safety is 
reduced. 

(c) Alarms are sent to a central sta- 
tion aboveground. Trained personnel can 
quickly notify miners of the danger and 
arrange for immediate correction of the 
situation, 

(d) With remote status of circuit 
breakers , it can be determined from the 



central station aboveground where power 
is energized or deenergized in a mine. 
This is useful on idle days to insure the 
proper equipment is on or off, 

(e) Parameters not required to be 
monitored, according to the CFR, can be 
monitored to provide increased safety. 

Production will improve because: 

(a) Downtime is reduced, 

(b) Inefficiencies are eliminated. 

Although it has only been discussed 
briefly in this report, remote control of 
power system components, such as circuit 
breakers , is possible and may be benefi- 
cial to the mine. 

The current state-of-the-art in sensor 
development is such that sensors are com- 
mercially available or the technology is 
available to monitor many parameters (ta- 
ble 2) that can increase mdne safety from 
power systems faults. 



Possible limitations of CCEIMMS are — 

(a) Large mines may require an exten- 
sive layout of sensors and cable. 

(b) Monitoring will not replace the 
need for visual, manual exams of the pow- 
er system, because a person may be able 
to detect potential hazards the CCRMMS 
cannot, such as deteriorating cable 
insulation. 

(c) Sensors installed at the face 
would have to be advanced with mining. 
Monitoring electric face equipment could 
be done at the load center, and cabling 
for the CCRMMS would advance with the 
load center. 

(d) Monitoring certain power system 
parameters may require an innovative en- 
gineering effort to develop the necessary 
sensors. 



REFERENCES 



1. Anderson, R. T. Reliability Design 
Handbook (Rome Air Development Center 
contract). IIT Res. Inst., Chicago, IL, 
Catalog RDH-376, March 1976, pp. 3-14, 

2. Bureau of National Affairs, Inc. 
Final Tables of Fatal and Nonfatal Mining 
Injuries. Mine Safety and Health Report- 
er; 1980, V. 3, No. 7, Aug. 26, 1981, 
p. 133; 1981, V, 4, No. 5, July 28, 1982, 
p. HI. 

3. Bureau of National Affairs, Inc. 
Selected Statistics on MSHA Citations. 
Orders of Withdrawal From 1979 Annual 
Report. Mine Safety and Health Reporter, 
V. 3, No. 9, Sept. 23, 1981, p. 173. 

4. Elswick, J. E. , and F. R. Schwam- 
berger. Electrical Hazards in Mining. 
Nat. Mine Health & Safety Academy, Beck- 
ley, WV, Safety Manual 9, undated, p. 3. 

5. Lyons, J. C. Underground Super- 
visory Control, Pres, at Mining 



Electro-Mechanical Maintenance Assoc. 
(ME-MMA) Annual Meeting, Wheeling, WV, 
April 1981; available for consultation at 
the Pittsburgh Research Center, Bureau of 
Mines, Pittsburgh, Pa. 

6. McDonald, L. B. , and W. H. Pomroy, 
A Statistical Analysis of Coal Mine 
Fire Incidents in the United States From 
1950 to 1977, BuMines IC 8830, 1980, 
42 pp. 

7. U.S. Mine Safety and Health Admini- 
stration (Department of Labor) . Injury 
Experience in Coal Mining. Selected Sta- 
tistics, 1979. MSHA IR 1122, 1980, p. 
249, 

8. Wright, H, A., R. Madden, and 
M. N. Rubin (Bolt Beranek and Newman, 
Inc., BuMines contract J0100039) . Guide- 
lines for Environmental Monitoring in 
Underground Coal Mines. Phase 1 Report. 
BuMines OFR 180-82, 1982, 128 pp.; NTIS, 
PB 83-147777. 



APPENDIX 



Table A-1, Current Mandatory Standards, 
lists a brief description of CFR 30, 
Part 75 requirements for each regulation 
discussed previously in the section, 



Analysis of Regulations. The rationale 
or hazard to be prevented by each regula- 
tion is also given. 



TABLE A-1. — Current mandatory standards 

CFR part and requirement^ Rationale 

75.513 — All electric conductors must be Prevent fires, 
large enough to carry the necessary load 
current without creating excessive heat, 

75.518 — All electric equipment, circuits, 
and 3-phase motors must be protected 
against short circuits and overloads. 

75.519 — Disconnecting switches must be 
installed in all main power circuits 
within 500 ft of the bottom of shafts, 
boreholes , and all other places through 
which main power circuits enter the 
mine. 



Prevent fires. 



Disconnect mine power in an emergency such 
as a fire or explosion or when a condi- 
tion exists that could result in a mine 
fire or explosion. 



Enable operation without danger of shock, 
fire, or faulty operation. 



Prevent methane ignitions and explosions. 



75.520 — Switches or controls on all elec- 
tric equipment must be safely designed, 
constructed, and installed. 

75.524 — The maximum level of ac or dc 
that exists between the frames of 2 
units of electric face equipment that 
come in contact with each other in work- 
ing places or in return air outby the 
last open crosscut must be <1 A. 

75.601 — Each conductor of all trailing Prevent fires, 
cables must be protected against short 
circuits by a device of adequate 
curent-interrupting capacity. 

75.702 — Methods other than conventional There are other safe methods of grounding 

grounding to limit the voltage that can such as electronic devices and diode 

appear between the frame of a machine grounding. Prevent shock, 
and ground to a safe value may be used. 

75.705 — Before maintenance work is per- Prevent shock, 
formed on underground high-voltage 
lines, they must be deenergized and 
grounded. 



See footnote at end of table. 



10 



TABLE A-1. — Current mandatory standards — Continued 

CFR P art and requirement^ Rationale 

75.706 — Unused underground power circuits Prevent fires. Rectifiers and transform- 
must be deenergized on idle days and ers should remain energized to combat 
shifts. Rectifiers and transformers may moisture, 
remain energized. 



75.800 — High-voltage circuits entering a 
mine must be protected by suitable cir- 
cuit breakers of adequate interrupting 
capacity which are equipped with devices 
to provide protection against undervolt- 
age, grounded phase, short circuit, and 
overcurrent. 



Prevent shock and fires. 



75.801 — The grounding resistor must be of Provide protection against shock and fire 
the proper ohmic value so that the volt- hazards, 
age drop in the grounding circuit be- 
tween the grounded side of the resistor 
and equipment frames be no more than 
100 V. The grounding resistor must be 
rated for the maximum fault current con- 
tinuously and insulated from ground for 
a voltage equal to the phase-to-phase 
voltage of the system. 

75.803 — High-voltage resistance grounded Prevent shock and fires, 
systems must include a fail-safe ground 
check monitor. 

75.808 — Disconnecting devices with visi- For isolation of faulted circuits and de- 
ble contacts must be installed at the energizing a circuit when not in use. 
beginning of all branch lines in high- 
voltage circuits. 

75.812 — Power centers, portable trans- .Prevent shock and fires, 
formers, and high-voltage cables, con- 
ducting power to these units, must be 
deenergized before they are moved from 
one location to another. 



75.900 — Circuit breakers equipped with 
devices to provide protection against 
undervoltage, grounded phase, short cir- 
cuit, and overcurrent must be installed 
to protect all low- and medium-voltage 
power circuits serving 3-phase ac 
equipment. 



Prevent electrocution and fires. 



See footnote at end of table. 



11 



TABLE A-1. — Current mandatory standards — Continued 
CFR Part and requirement^ Rationale 



75.902 — Low- and medium-voltage resist- 
ance grounded systems must include a 
fall-safe ground check circuit. 



Prevent shock, 



75.903 — Disconnecting devices, which pro- Prevent shock, 
vide visual evidence that the power is 
disconnected, must be installed in con- 
junction with circuit breakers. 



75.906 — Trailing cable for mobile equip- 
ment must contain at least one ground 
conductor (cross-sectional area > 1/2 
the power conductor) and an insulated 
conductor for the ground continuity 
check circuit. 



Prevent electrocution. 



75. 1000 — Cutout switches must be in- Facilitate removal of power during emer- 
s tailed in trolley wires and trolley gencies , idle periods, and at times when 
feeder wires at Intervals of 2,000 ft or maintenance work is performed, 
less and near the the beginning of all 
branch lines. 



75.1001 — Automatic current interrupting Prevent fires, 
devices must be Installed to protect 
trolley wires and trolley feeder wires 
against damage by over current. 



'u.S. Code of Federal Regulations, Title 30. 



irU.S. GOVERNMENT PRINTING OFFICE: 1983-605-015/60 



INT.-BU.OF MINES, PGH., PA. 27033 




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